1. Technical Field
The present invention relates to systems for deflecting electron beams or streams of other charged particles. The invention also includes a method of particle beam deflection.
2. Prior Art
In the control of monochromatic electron beams for electron microscopy or other purposes, it is usual to pass the electron beam through a fixed spatial arrangement of deflecting plates across which a variable electrostatic field is produced in dependence on the degree of deflection required. Therefore such systems require the provision of means for producing electrostatic fields of variable magnitude. It will also be appreciated that the degree of deflection of the beam will depend on the amount of time spent by the individual particles between the plates. In present systems this time depends solely on the velocity of the particles being deflected, which is related to their energy.
In one application of an electron microscope to study the performance of a working integrated circuit chip it is required to "illuminate" the chip with the electron beam in a stroboscopic manner. Thus the chip is observed at times separated by a time period equal to the operating cycle time of the circuit, and the circuit is always in the same electrical state. The electron beam is said to be "blanked" in between observations. To achieve this blanking the beam is rapidly deflected away from the chip by applying an electric field to deflecting plates surrounding its path.
Particular difficulties are experienced with the present systems when it is desired to make provision in the same structure for the rapid deflection of either a beam of high velocity particles or a beam of low velocity particles. A set of plates sufficiently long to provide adequate deflection for a high velocity beam will be traversed so slowly by a beam of low velocity particles that the transition between the blanked and unblanked conditions will be unacceptably slow, and, moreover, it is necessary to change the voltage applied to the plates for deflecting low velocity particles in order to prevent the particles striking the plates and so impairing the future performance of the system.